1. Field of the Invention
The present invention relates to a transmission system capable of accommodating both a synchronous communication system such as SONET/SDH and an asynchronous communication system such as TCP/IP, and in particular relates to a transmission system which is capable of transmitting synchronous-system data without transmission delays or fluctuations thereof, while maintaining path processing for asynchronous-system data.
2. Description of the Related Art
In a synchronous communication system such as SONET (Synchronous Optical NETwork)/SDH (Synchronous Digital Hierarchy), a high-speed transmission standard based on optical fiber, time-division switches multiplex synchronous data to perform routing. And in an asynchronous communication system such as TCP/IP, packet switches switch packets, which are asynchronous data units, to different destinations. For synchronous data the same destination is always specified and circuits are occupied, but for asynchronous data (packets) the destination of each packet is specified, and circuits are not occupied. Further, synchronous data is required to have a constant delay time, but in the case of asynchronous data, adjustment is performed on the end-user side even when there is fluctuation in delay times, so that no problems arise.
When implementing communication in which the two types of data, which are synchronous data and asynchronous data, are intermixed, in the past the following methods have been proposed. A first method of the prior art is a method in which asynchronous data is all converted into a frame format of synchronous data, and all data is handled as synchronous data; a second method of the prior art is a method in which, in contrast to the first method, all synchronous data is converted into an asynchronous data format, and all data is handled as asynchronous data.
FIG. 1 shows a transmission system with a ring configuration. The ring-shape transmission path 1 is a high-speed data transmission path such as SONET/SDH or 10-Gigabit Ethernet (a registered trademark); in FIG. 1, a framer 10 (hereafter sometimes called a “node” on the transmission path) to synchronized transmitted data into prescribed frames is connected in the ring-shape transmission path 1. The high-speed transmission path 1 is not limited to a ring shape, but may have a mesh shape or another network configuration.
The framer 10 comprises a reception unit 11, which receives data transmitted on the bidirectional transmission path 1, and a transmission unit 12, which transmits data on the transmission path. The reception unit 11 is an optical-electrical conversion unit which converts received optical signals into electrical signals. The transmission unit 12 is an electrical-optical conversion unit which converts electrical signals into optical signals, which are transmitted. The frame synchronization and OH processing unit 13 drops all received data to a path selection processing unit (switch) 20, and the path selection processing unit (switch) 20 extracts data addressed to its station, and transfers the data to the low-speed interface 30. The path selection processing unit 20 transfers data addressed to other stations and added from the low-speed interface 30 to the frame synchronization and OH processing unit 13. The frame synchronization and OH processing unit 13 synchronizes transmitted data in frames according to the transmission specifications of the ring transmission path, and processes monitoring control information contained in the overhead (OH).
FIG. 2 shows an example of the configuration of a transmission system for the case in which asynchronous data is converted into synchronous data and transmitted (the first method of the prior art). The configuration example of FIG. 2 shows the configuration of the dashed-line units in FIG. 1. Synchronous data is handled, and so the path selection processing unit 20 is a time-division switch 20B.
When the synchronous data is SONET/SDH VC (Virtual Container) frames, asynchronous data, which is for example a MAC frame, must be converted into synchronous data VC frames, as shown in FIG. 2; the MAC→VC frame conversion unit 31A of the low-speed interface 30A on the side of the asynchronous communication system converts variable-length MACs frame into fixed-length VC frames, and transmits the result as synchronous data to the time-division switch 20B. VC frames are time-division multiplexed and sent to the framer 10. When the transmission specification of the ring transmission path 1 is also SONET/SDH, the synchronous data is transferred to the time-division switch 20 as-is, without frame conversion by the low-speed interface 30B on the synchronous communication system side, and after frame synchronization by the framer 10, is transmitted to the transmission path 1.
The time-division switch 20B extracts data addressed to its own station from all the synchronous data (including frame-converted asynchronous data) dropped from the framer 10, and based on the data destinations, outputs the data in the destination directions on the low-speed side. Of the extracted synchronous data, that data converted from asynchronous data is output to the low-speed interface 30A on the asynchronous communication system side, which is the destination direction, and the VC→MAC frame conversion unit 32B converts the received VC frames for the synchronous system back to MAC frames for the asynchronous communication system. Further, among the synchronous data addressed to its own station, synchronous data excluding the data converted from asynchronous data (data which has been synchronous data from the beginning) is transferred to the low-speed interface 30B on the synchronous communication side which is the destination direction, and is output without change as synchronous data.
This first method of the prior art performs relaying using a time-division switch (relay station) 20B, and so can minimize the transmission delay time (with a delay of one frame's worth only) and enable highly reliably relaying. However, originally it is expected that asynchronous data will have priorities assigned to packets for processing according to the type of service. That is, asynchronous data differs from synchronous data in that the need arises for relaying via integrated circuits which perform complex route processing and priority processing. Such functions cannot be incorporated into a time-division switch, and so in the end only simple functions for transferring data can be used with asynchronous data, and the advantages of asynchronous data are largely lost.
FIG. 3 shows an example of the configuration of a transmission system for a case in which synchronous data is converted into asynchronous data and transmitted (the second method of the prior art). The configuration example of FIG. 3 shows the configuration of the dashed-line units in FIG. 1. Asynchronous data is handled, and so the path selection processing unit 20 is a packet switch 20A.
As shown in FIG. 3, synchronous data must be converted into asynchronous data, and the VC→MAC frame conversion unit 32B of the low-speed interface 30B converts synchronous data, which are VC frames, into MAC frames, which are asynchronous data frames, and transfers the data as asynchronous data to the packet switch 20A. The MAC frames are subjected to statistical multiplexing, and are sent to the framer 10. The asynchronous data is transferred as-is to the packet switch 20A without frame conversion from the low-speed interface 30A of the asynchronous communication side, and are sent to the framer 10. The framer 10 synchronizes asynchronous data from the packet switch 20 in frames according to the transmission specification of the ring transmission path (for example, LAN PHY), and transmits the data.
The packet switch 20A extracts data addressed to its own station from all the asynchronous data (including frame-converted synchronous data) dropped from the framer 10, and based on the destinations of the data, outputs the data in the low-speed side destination directions. Of the extracted asynchronous data addressed to its own station, that converted from synchronous data is output to the low-speed side interface 30B of the synchronous communication system, which is the destination direction, and the MAC→VC frame conversion unit 32A returns the received MAC frames for asynchronous communication into VC frames for synchronous communication and outputs the frames. Of the asynchronous data addressed to its own station, the asynchronous data excluding that converted from synchronous data (data which had been asynchronous data from the beginning) is transferred to the low-speed side interface 30A of the asynchronous communication side, which is the destination direction, and output as-is as asynchronous data.
Because packet switching is used in this second method of the prior art, the advantages of asynchronous data in the above first method of the prior art can be utilized; but because even synchronous data is transferred by packet switching, there are always fluctuations in the transfer time of synchronous data. Further, because of the complexity of relay processing, reliability is reduced, and in addition this method effectively entails storage communication, so that transmission delay times tend to be large.
In the configuration of this second method of the prior art, processing of synchronous data as top-priority packets is conceivable as a method of minimizing the delay time for synchronous data. However, if this method is used, all unrelated top-priority packets (frame-converted synchronous data) not addressed to its own station are sent to the packet switch, resulting in reduced operating capacity of the packet switch, so that the problems of sharply reduced communication speed of asynchronous data not subjected to prioritized processing and increased discarding of packets arise.
In addition, a transmission method is conceivable which employs a configuration in which a time-division switch for synchronous data and a packet switch for asynchronous data are positioned in parallel (hereafter called a third method of the prior art).
FIG. 4 shows an example of the configuration of a transmission system for the case in which synchronous data and asynchronous data are processed and transmitted in parallel (the third method of the prior art). The multiplexing unit 50 multiplexes synchronous data added from the time-division switch 20B and asynchronous data added from the packet switch 20A. In the ring transmission path, it is necessary to unify the frame format as either synchronous data or asynchronous data; for example, in FIG. 4, asynchronous data is converted into the frame format for synchronous data. When the synchronous data is in SONET/SDH frames, and the high-speed ring transmission path is also SONET/SDH, the synchronous data must be converted into SONET/SDH.
The frame conversion unit 60 performs frame conversion in order to cause the MAC frames, which are asynchronous data, to be compatible with the SONET/SDH format, so that synchronous data and asynchronous data can be multiplexed in the multiplexing unit 50. For example, frame conversion is performed by encapsulating MAC frames so as to enable SONET/SDH transmission, based on GFP (Generic Framing Procedure) and LCAS (Link Capacity Adjustment Scheme) methods.
Conversely, when the specifications of the ring transmission path 1 are such as to enable unmodified handling of MAC frames of asynchronous data (for example, the LAN PHY specification), the frame conversion unit 60 converts VC frames of synchronous data flowing in the transmission path direction into MAC frames, and synchronous data flowing in the direction of the low-speed side are returned from MAC frames to VC frames.
Such a third method of the prior art requires placement of both a packet switch and a time-division switch, and is disadvantageous in terms of both circuit scale and implementation efficiency; in addition, due to the different frame formats of data handled by the packet switch and the time-division switch, the following problems arise.
That is, because it is impossible to distinguish between data which is to be sent to the packet switch and data which is to be sent to the time-division switch, all the data dropped from the framer 10 must be transferred to both the packet switch and to the time-division switch. The need arises for each switch to process all data. Hence the processing load on both switches becomes excessive; in particular, the problems of the above-described second method of the prior art remain. That is, in the packet switch, the problems of greatly diminished communication speed of asynchronous data and of an increase in the discarding of packets occurs.
Japanese Patent Laid-open No. 2003-324453 discloses a transmission device which reads the arrival time order of asynchronous data from a buffer and adds a plurality of identifying tags to transmit data, to convert asynchronous data into synchronous data and perform multiplexing, transmission and reception.
Further, Japanese Patent Laid-open No. 5-37560 discloses a transmission device in which, when data equivalent to a packet length is accumulated in a transmission buffer, a time stamp based on an asynchronous clock is added and the data transmitted, so that asynchronous data can be converted into synchronous data and transmitted and received without being affected by delay fluctuations of asynchronous data. Japanese Patent Laid-open No. 2003-324453 and Japanese Patent Laid-open No. 5-37560 described inventions in which asynchronous data is converted into synchronous data and transmitted.
An object of this invention is to provide a transmission system which is capable of multiplexing and transmitting synchronous data and asynchronous data, while maintaining the communication quality required of synchronous data and of asynchronous data respectively.